Dispensers of the aforementioned type are known for example from U.S. Pat. No. 6,695,246.
In for example the dispenser according to U.S. Pat. No. 6,695,246, the sensor control circuitry uses active IR (i.e. both IR emission and detection) to control sensing of the presence of a possible user. The IR is emitted in pulses. In the active IR mode, the presence of an object (i.e. a possible user) can be detected within a detection zone of about 12 to 24 cm from the dispenser and upon said detection operates a motor to dispense a hand towel to a user. One IR receiver and one IR emitter are located behind a front cover of the dispenser and each is mounted in a respective tube, the tubes being placed adjacent one another. By this arrangement the detection distance is kept short (between about 12 to 24 cm) so that objects which are outside the detection zone do not lead to undesired and unintentional dispensing. Likewise the object must be in the correct position and at a correct angle otherwise the tubes will prevent IR from being reflected back and collected by the receiver. Thus, while the possibility of undesirable reflections from other surfaces or the like is reduced, the sensor system requires fairly accurate positioning of the hand to effect operation. When an object is detected, the microprocessor activates the motor to dispense a towel if two scans with sufficient reflected IR are received the IR sensing control system. In order to operate the motor, the IR sensor system uses a background light level reference circuit which provides a reference voltage VB, related to the level of background light and against which a voltage VA from the IR sensor is compared. When voltage VA is greater than voltage VB the motor may operate to dispense a hand towel. This provides an automatic compensation of background light level so that the signal picked up by the IR receiver must be raised to a higher level in order that a user is detected.
In the aforementioned dispenser, although a certain degree of compensation is made for background light levels which can take account of some conditions, problems will arise in such a device due to temporary effects of high IR changes due to atmospheric conditions, in particular when background IR is very low, since only very small changes in reflected IR can cause dispensing to occur even when not required.
Likewise, the problem would also be encountered that as background IR levels generally increase to a high level, a user's hand becomes harder to distinguish against the background IR because the increased level of IR due to the IR reflection from a user's hand, when the hand is in front of the dispenser, may be approximately at the same level as the background IR, or due to the temporary blocking of the high level background IR, the presence of a hand can even reduce the level to below that of background IR received in the IR sensor, such that the hand in many cases is not reliably detected.
Also, a user's hand which is not detected or is not correctly positioned with respect to the small area of detection of the sensors on the dispenser, i.e. in the small range of area detected and thus which does not immediately activate hand towel dispensing, will often be tempted to touch the dispenser to try and cause dispensing in the belief that touching of the casing close to the sensors is required, despite any written notices which the dispenser may contain in this regard. This is particularly the case because the user's hand is already at the height of the dispenser as in the aforementioned document. This can result in lack of hygiene when several users consecutively touch the dispenser.
Further, while the IR emitted intensity from the emitter is seemingly constant in the aforesaid document (apart from possible variations to low battery voltage), such a construction when relying on battery power from batteries (rather than solar cells) often involves using unnecessarily high power.
The present invention has as one of its objects, to provide an improvement to the active IR detection to take account of background IR changes.
A further object is to improve the possibility of better hygiene.
A further object is to minimize power consumption of the device at certain times by taking into account the background IR level.
Further objects of the invention will be apparent upon reading this specification.